1. Field of the Invention
The present invention relates to an automatic control circuit for a power amplifier for use in digital cellular mobile telephone systems and portable telephones.
2. Description of the Prior Art
FIG. 6 is a diagram of an automatic control circuit for a power amplifier of the prior art which is used for burst transmission in digital cellular mobile telephone systems and portable telephones. In FIG. 6, reference numeral 1 represents a baseband signal, 2 an oscillator for generating a carrier wave, 3 a modulator for modulating the carrier wave 2 with the baseband signal 1, 4 a radio frequency signal output from the modulator 3, and 5 a power Amplifier for amplifying the radio frequency signal 4 and having a gain control terminal 5a. Reference numeral 6 represents a radio frequency output signal (referred to as "RF output signal" hereinafter) amplified by the power amplifier 5, 7 a detector for detecting the envelope waveform of the RF output signal, 8 a reference signal voltage generator for generating the waveform of a reference signal voltage 31, 9 a comparator for detecting the difference between the waveform detected by the detector 7 and the waveform of the reference signal voltage 31, and 10 a low-pass filter for filtering an error signal 32 output from the comparator 9 and supplying the error signal 32 to the gain control terminal 5a of the power amplifier 5. The reference signal voltage generator 8 comprises a D/A converter 12 for converting digital data into analog data to output the reference signal voltage 31, a waveform generating ROM 13 for storing data 33 to be supplied to the D/A converter 12, a counter 14 for specifying an address in the ROM 13, and a RAMP controller 15 for outputting a signal for controlling a transmission waveform at the start and end of burst transmission to the counter 14. Reference numeral 16 represents a transmission control signal for controlling the counter 14 and the ROM 13, and 17 a power level control signal for controlling the ROM 13.
A description is subsequently given of the operation of the prior art. The baseband signal 1 is modulated by the modulator 3 together with the carrier wave from the oscillator 2 to become the radio frequency output signal 6 (referred to as "RF output signal" hereinafter). This RF output signal 6 is amplified by the power amplifier 5 (abbreviated as PA hereinafter), and transmitted through an isolator, a duplexer and an antenna. For the purpose of effecting an optimal and stable transmission, the waveform of the RF output signal 6 amplified by the PA 5 is detected by the detector 7, and this detected output and the reference signal voltage 31 from the reference signal voltage generator 8 are compared by the comparator 9 to output an error signal voltage 32 which is then fed back to the gain control terminal 5a of the PA 5 through the low-pass filter 10. Furthermore, the reference signal voltage generator 8 operates in such a manner that the waveform-generating ROM 13 and the counter 14 are activated by the transmission control signal 16 at the start of transmission and a CLK signal is then inputted into the counter 14 so that the counter 14 begins to count and specifies the address of the waveform data 33 prestored in the waveform generating ROM 13. The waveform-generating ROM 13 stores waveform data 33 for each transmission power level and switches data in response to a power level control signal 17. When an address in the waveform-generating ROM 13 is specified, the waveform data 33 at the specified address is input into the D/A converter 12 to output the waveform of the reference signal voltage 31. For the purpose of preventing the expansion of a spectrum caused by drastic changes in the waveform of the RF output signal 5, such as the steep rises and falls that occur in the burst transmission mode, the counter 14 is controlled by the RAMP controller 15 upon input of the CLK signal at the start and end of transmission to generate the reference signal voltage 31 by which rises and falls in the waveform of the RF output signal 6 can be smoothed.
Since there is an offset voltage in the comparator 9 used in the automatic control circuit for a transmission power amplifier of the prior art, an undesirable charge is accumulated in the capacitor of the low-pass filter 10 at the time of non-transmission in the burst transmission mode. An undesirable charge is accumulated in the capacitor especially when the automatic control circuit for the power amplifier is turned off at the time of non-transmission for the purpose of power saving. If transmission starts in this state, the gain control of the power amplifier 5 cannot be performed precisely as response of the low-pass filter 10 is unstable, resulting in the occurrence of undesirable changes in the waveform of the RF output signal 6 such as deformed rises and overshoots.